In a modern petroleum refinery several process units generate hydrocarbon by-products of low commercial value. Side streams comprising lower molecular weight paraffins are generally low in value and often burned as a source of fuel.
Efforts to upgrade hydrocarbon streams containing C.sub.2 -C.sub.10 paraffins have included contacting the paraffins under high severity dehydrocyclization conditions with a crystalline shape selective medium pore siliceous acid catalyst in a fluidized bed reaction zone to obtain aromatic hydrocarbons. Such an operation provides a convenient one-step route for producing an aromatics-rich gasoline with a relatively high octane number.
Givens U.S. Pat. No. 3,827,968 (et al.) discloses a two-stage operation wherein a mixed feed containing paraffins and olefins is upgraded in the absence of added hydrogen to a highly aromatic gasoline product. In a first stage oligomerization reaction, olefins are upgraded to higher molecular weight liquid hydrocarbons under relatively mild conditions with a catalyst having the structure of ZSM-5. In a second stage, said liquid hydrocarbons are converted to an aromatic product. A gas phase highly paraffinic stream is withdrawn as by-product from the first stage.
Numerous references describe the conversion of light aliphatic hydrocarbons to aromatic hydrocarbons. For instance, J. Eng U.S. Pat. No. 2,992,283 describes the conversion of propylene to a variety of higher molecular weight hydrocarbons using a treated crystalline aluminosilicate as the catalyst. C. M. Detz U.S. Pat. No. 4,347,394 (et al.) describes the conversion of C.sub.5 + hydrocarbons to aromatics using a nonacidic zeolite supporting a platinum compound. P. J. Conn U.S. Pat. No. 4,329,532 (et al.) describes the conversion of C.sub.4 - olefins or mixtures of olefins and paraffins to aromatic hydrocarbons. The catalyst comprises a crystalline silicate having a specified composition, crystallite size range, and X-ray diffraction pattern.
E. E. Davies U.S. Pat. No. 4,180,689 (et al.) describes the conversion of C.sub.3 -C.sub.8 aliphatic hydrocarbons to aromatic hydrocarbons in a process which employs a catalyst comprising gallium supported on an aluminosilicate. L. D. Rollman U.S. Pat. No. 3,761,389 (et al.) describes an improved process for converting hydrocarbons to aromatics over a catalyst having the structure of ZSM-5. The improvement resides in the use of two reaction stages in series, with the first being at more severe operating conditions. P. C. Steacy U.S. Pat. No. 4,528,412 also describes catalyst, reaction zone operations and product recovery methods for dehydrocyclodimerization processes.
Jensen U.S. Pat. No. 4,642,402 describes a process for the conversion of light aliphatic hydrocarbons to aromatics. The improvement resides in recycling benzene recovered from the reaction zone effluent to the reaction zone.
A review of dehydrocyclodimerization is presented at page 191 of Vol. 18, No. 2, 1979 issue of "Industrial Engineering Chemistry: Process Design and Development".
The primary objective of motor gasoline manufacture and of the present invention is to maximize production of high-octane gasoline. It is widely known that yield and octane increase are inversely related for a given hydrocarbon feedstream. Consequently, the effectiveness of a particular upgrading process can be expressed in terms of octane-barrels per day. The index "octane-barrels per day" is defined as follows: ##EQU1##
The primary object of the present invention is to maximize the octane-barrels per day produced by the conversion of an aliphatic hydrocarbon feedstock. The aromatics liquid product usually contains at least 70% aromatics with a distribution of aromatics product such that toluene usually predominates with significant quantities of benzene and C.sub.8 aromatics together with somewhat lower quantities of C.sub.9 + aromatics. From a gasoline additive product value point of view, the lower alkyl substituted aromatics are to be preferred as compared to the non-substituted benzene because they have as high octane values and greater weight and volume per octane number. The present invention provides a process for increasing the production of high octane gasoline by shifting the product distribution from non-substituted benzene to lower alkyl substituted aromatics, for decreasing production of light C.sub.3 - byproducts, and for balancing the relative yield of aliphatic and aromatic product to maximize product octane-barrels.